Impelled by environmental concerns and increasing governmental regulations on the volatile organic content of inks and coatings, the applications of water-based flexographic and rotogravure inks are increasing in the ink industry. A typical water-based ink system is formulated by the addition of a binder resin (usually acrylic polymer lattices) to a grind base. Grind bases are prepared by using grinding resins to disperse pigments. Pigments are crystalline solids composed of agglomerates, aggregates, and primary particles which vary in size from 0.02 to 0.5 microns. During the grinding process, agglomerates and aggregates are broken down into primary particles which possess strong tendencies to reassociate in an ink. The finer the particle size of the pigment, the greater the color strength, but the more difficult the pigment becomes to disperse. Grinding resins help prevent the particles from reassociating by increasing both the electrostatic and steric repulsion between pigment particles.
An ink formulator must consider the compatibility of the various ink components when selecting the grind resins to be used. Grind bases (i.e., pigment dispersions) are let down with a variety of alkali-soluble resins or alkali-insoluble resin emulsions to achieve the properties desired for the end use of the ink. If the grind resins and the let-down resins are not compatible, the result may be pigment flocculation, viscosity increase, loss of color strength, and other problems.
Currently, the rosin resin FILTREZ.RTM. 5014 (manufactured by Akzo Chemicals, Inc.) is preferred in the alkali-soluble grind resin market. This resin has the ability to produce stable low viscosity grind bases and inks for water-based flexographic printing. FILTREZ 5014 also yields excellent color development when grinding pigments and contributes to the toughness and water resistance of finished inks. However, both the viscosity and the stability of grind bases and inks produced with the aid of commercially available polyester resins are highly dependent on the pigment-to-resin ratio, pigment loading, and pH-value. As grind bases must remain stable and fluid to be applicable, this pH dependence has remained a problem associated with the use of these resins.
In commonly assigned U.S. Pat. No. 5,066,331, Dr. G. Frederick Hutter describes the production of water-soluble resins having superior adhesive properties for use in formulating packaging inks. These resins are produced by reacting fumarated or maleated rosin with a compound containing two secondary amine groups and no primary amine groups. As Dr. Hutter states in the patent (Col. 2, lines 59-66), it is essential to his invention that primary amine groups not be utilized in the reaction with the modified rosin. While these resins make excellent binder resins for water based packaging inks, the resins are less successful when employed as grinding resins. Indeed, when one attempts to employ these resins as grinding resins, the resulting pigment dispersions are thixotrophic and extremely difficult to handle and utilize.
At the present time, the majority of water-based printing inks are produced at pH values between 8.5 and 10.5. It would be advantageous to ink formulators to have a resin which is stable over this entire range. Therefore, it is the object of this invention to have an alkali-soluble resin which may be used to prepare stable, low viscosity pigment dispersions over a wide range of pH values while maintaining excellent physical properties in the finished inks.